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  Java Practices -> Tag or marker interfaces
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<div class='page-title'>Tag or marker interfaces</div>

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<br>A <i>tag interface</i> (also called a <i>marker interface</i>) is simply an interface with no methods. 
Some examples of tag interfaces from the JDK :
<ul>
 <li><a href='http://java.sun.com/javase/6/docs/api/java/io/Serializable.html'>Serializable</a>
 <li><a href='http://java.sun.com/javase/6/docs/api/java/util/EventListener.html'>EventListener</a>
</ul>

<P>Why define an interface with no methods? 
Since there are no methods, a tag interface can never define behavior, at least not in the typical sense.
However, even though it has no methods, a tag interface always carries <i>type information</i>.
In some cases, type information itself can be used to solve a problem. 
<P>For example, Java's serialization mechanism requires an object to implement Serializable before it will serialize it.
As stated in its <a href='http://java.sun.com/javase/6/docs/api/java/io/Serializable.html'>javadoc</a> :

<P><i>The serialization interface has no methods or fields and serves only to identify the semantics of being serializable.</i> 

<P>Tools will often use <tt>instanceof</tt> or reflection to inspect objects, to see if they implement a given tag interface. 
But this isn't the only way a tag interface can be useful.
There are some common cases for which you might consider defining your own tag interface, even without any corresponding use of <tt>instanceof</tt> or reflection :
<ul>
 <li>immutable objects (or immutable classes, if you prefer)
 <li>singleton classes
</ul>

The advantages of doing so are :
<ul>
 <li>it emphasizes important, high-level aspects of a class, not otherwise expressed in code.
 <li>for both human readers and tools, it allows quick identification of classes having specific properties.
 <li>the javadoc of the tag interface is a natural home for documenting all the characteristics of items that implement the given interface.
</ul>


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<div class='topic-section'>See Also :</div>
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  <a href='TopicActiond838-2.html?Id=29'>Immutable objects</a> <br>
 
  
  <a href='TopicAction4934-2.html?Id=46'>Singleton</a> <br>
 
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